An image projection system includes components used to impose an image on an incident beam of light and to produce an output image beam. A projection system generally includes an image forming component, or imager, such as a liquid crystal on silicon (LCOS) device or a digital micromirror device (DMD). A DMD is a semiconductor device that selectively reflects illumination light to produce imaging light, and includes a multitude of microscopically small, individually controllable mirrors laid out in a matrix on a plane.
In a typical digital light processing (DLP) projection display using a DMD (available from Texas Instruments), light from a light source strikes the imager and is modulated by the micromirrors at each pixel. Each movable mirror can direct the light reflected from its surface so that it goes towards the projection lens system and screen, or off into a reject state where it is blocked from getting to the screen.
One architecture for a DLP projection engine uses a total internal reflection (TIR) prism to separate the illumination light from the imaging light reflected from the DMD imager. A TIR prism has a face that is close to the critical angle of reflection, i.e., it has a face at which light at some angles undergoes total internal reflection and light at other angles passes through the face. This distinction in how light is directed based upon small differences in angle is exploited in a TIR prism to manipulate light in the limited volume available in practical DLP devices.